Method and apparatus for estimating the location of a signal transmitter

ABSTRACT

In accordance with the principles of the present invention, a beaconing device transmits a radio signal that can be detected by a plurality of wireless receivers positioned at various known locations. The receivers form nodes of a wireless network that further includes a control node with which the receivers can communicate. Each receiver that receives the signal from the beaconing device records the signal strength at which it receives the signal from the beaconing device and sends that information to the control node. The control node processes the signal strength information received from all of the receivers and uses it to estimate the location of the beaconing device. Particularly, the controller solves an optimization problem for the coordinates of the beaconing device by determining the set of coordinates for the beaconing device that minimizes the squared error over all of the receivers that receive the signal from the beaconing device between (1) the Euclidian distance between the known coordinates of the receiving device and the estimated coordinates of the beaconing device and (2) the estimated distance between the beaconing device and the particular receiver based on signal strength at that receiver.

FIELD OF THE INVENTION

The invention pertains to a technique for estimating the location of a transmitter based on information from a plurality of receivers.

BACKGROUND OF THE INVENTION

There are many situations in which it is desirable to determine the location of a person or object by sensing electromagnetic radiation from the object. Radar would be one such example. With radar, the electromagnetic radiation is not radiation originating from the object or person but is merely reflected off the object or person. In other circumstances, the object or person may be equipped with a positioning device such as a GPS unit that includes a transmitter for transmitting the coordinates of the person or object via radio waves which can be received at a receiver. Such techniques are known in the military for locating lost soldiers.

These techniques, however, require relatively expensive equipment. They are all are circumstances in which it is desirable to track the location of an object or a person but for which it would be difficult to justify the cost of these types of location systems.

For instance, there are circumstances under which it may be necessary or advisable to track the movements of one or more persons within a relatively well-defined space, such as a home, hospital, or prison. As an example, elderly or infirm persons that live alone or in a nursing home may need frequent or even relatively constant monitoring by caregivers. In order to reduce the staffing needs for monitoring and caring for persons in such situations and/or to reduce the burden on other family members or household members, it would be desirable to automate to the extent possible the monitoring of such persons.

For instance, in many instances it may be desirable to monitor the movement of a person about a house so as to know if that person is going to the bathroom or using the kitchen on a normal basis. Alternately, it may be desirable to track the movement of a person in order to assure that the person is moving on a regular basis and not incapacitated or otherwise unable to move.

In other circumstances such as institutional situations like nursing homes or hospitals, it may simply be advantageous to know the whereabouts of individuals so that they can be located for purposes of being provided medications or other care or simply to find them when they are missing.

However, the cost of location systems such as radar and GPS would not be practical in such circumstances for many reasons, not the least of which is the cost. Particularly, both radar and GPS systems probably would not work effectively indoors because of walls and ceilings that would block the radar signals as well as access to the GPS satellites.

Accordingly, it is an object of the present invention to provide an improved method for tracking individuals.

It is another object of the present invention to provide an improved apparatus for tracking individuals.

It is a further object of the present invention to provide a new method and apparatus for determining the location of a beaconing device relative to a plurality of signal receiving devices that receive a signal from the beaconing device.

It is yet a further object of the present invention to provide a method and apparatus for accurately predicting the location of a beaconing device based on received signal strength measurements of a signal obtained by various receivers positioned in various locations.

SUMMARY OF THE INVENTION

In accordance with the principles of the present invention, a beaconing device transmits a radio signal that can be detected by a plurality of wireless receivers positioned at various known locations. Preferably, the signal transmitted by each wireless beaconing device comprises an ID that uniquely identifies the beaconing device so that the technique can be used to determine the locations of multiple beaconing devices simultaneously. However, in environments in which there is only one beaconing device, a unique ID may be omitted. In a preferred embodiment of the invention, the receivers form nodes of a wireless network that further includes a control node. The receivers can communicate with the control node.

The beaconing device transmits its signal. Each receiver that receives the signal from the beaconing device records the signal strength at which it receives the signal from the beaconing device (as well as the ID of the beaconing device, if so adapted) and sends that information to the control node. The control node processes the signal strength information received from the receivers and uses it to estimate the location of the beaconing device. Particularly, the controller performs an algorithm to solve an optimization problem for the coordinates of the beaconing device by determining the set of coordinates for the beaconing device that minimizes the squared error over all of the receivers that receive the signal from the beaconing device between (1) the Euclidian distance between the known coordinates of the receiving device and the estimated coordinates of the beaconing device and (2) the estimated distance between the beaconing device and the particular receiver based on signal strength at that receiver.

Over time and a plurality of signals transmitted from the beaconing device at intervals, the controller can track the location and movement of a person and provide that information through an interface device, such as a computer monitor, to caregivers. In other embodiments, the controller can further process the data collected from the receivers and analyze it for particular traits that might indicate that the individual is injured or otherwise having difficulty. Such traits might include lack of movement for an extended period of time, failure to go to a particular place in the household, such as the bathroom, on a reasonably regular basis, or too frequent visits to a particular place in the household, such as the bathroom.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the basic components of a tracking system incorporating the principles of the present invention.

FIG. 2 is a schematic plan view of a household incorporating a tracking system in accordance with the principles of the present invention.

FIG. 3 is a block diagram illustrating the components of one of the wireless beaconing devices of FIG. 1 in accordance with the present invention.

FIG. 4 is a block diagram illustrating the components of one of the wireless receivers of FIG. 1 in accordance with the present invention.

FIG. 5 is a block diagram illustrating the components of the controller of FIG. 1 in accordance with the present invention.

FIG. 6 is a graph illustrating distance as a function of received signal strength.

FIG. 7 is a diagram illustrating the relative positions of a beaconing device and six anchor devices for exemplary purposes.

DETAILED DESCRIPTION OF THE INVENTION

U.S. patent application Ser. No. ______ (attorney docket number H0011714-0760/Outside Counsel Docket No. P 31037 USA), entitled Method and Apparatus for Tracking Persons, which is incorporated fully herein by reference, discloses a method and apparatus for tracking person or objects. Particularly, one or more individuals carries a small, light wireless beaconing device that sends out a low-power radio signal that can be detected by a plurality of wireless receivers positioned at various locations in a household (or other relatively small area). The beaconing device may be contained within an article easily worn on the person's body, such as a piece of jewelry, a watch, or a key fob. Preferably, the receivers form nodes of a wireless network that further includes a control node. The receivers communicate with the control node. Each wireless receiver that receives the signal from the beaconing device records the signal strength at which it receives the signal from the beaconing device and sends that information to the control node. The control node processes the information received from the receivers and uses it to estimate the location of the individual. Over time and with the information derived from receipt of a plurality of beacon signals by the plurality of receivers, the control node can determine the movements of the individual and evaluate that information to assess whether the individual requires attention from a caregiver. The aforementioned patent application notes that wireless home security systems already contain much of the hardware for practicing the invention disclosed in that application, such as lightweight and small wireless transmitters and fixed receivers adapted for household use.

The present invention pertains to a technique that can be employed in the system disclosed in the aforementioned patent application for accurately estimating the location of the beaconing device based on the received signal strength data collected from the plurality of receivers. While the present invention was particularly developed for this type of application, it should be clear that it can be used in an almost limitless variety of other applications. Particularly, it can be used to estimate the location of virtually any transmitter that is detected by multiple receivers, wherein at least one of those receivers can determine received signal strength. Thus, the principles of the present invention have applicability in military scenarios, asset tracking scenarios (e.g., a warehouse), person tracking scenarios (prisons, hospitals), etc.

Using the location information gathered from the anchor devices over a period of time, the controller can track the location and movement of a person and provide that information through an interface device, such as a computer monitor, to caregivers. In other embodiments, the controller can further process the data collected from the receivers and analyze it for particular traits that might indicate that the individual is injured or otherwise having difficulty. Such traits might include lack of movement for an extended period of time or too frequent or too few visits to the bathroom.

Home security systems are widely available in which a plurality of the detector devices, such as door and window monitors designed to detect the opening of a door or window (such as by the loss of electrical continuity between two electrodes in which one is mounted to the moveable window or door and the other is mounted to the stationary frame of the window or door) are coupled to one or more control panels from which the owner of the residence can control the security system. In addition, the system typically also includes an alarm node that will sound an alarm in the event of certain circumstances (e.g., a window being opened when the system is enabled). Often, the system is also hooked up to the telephone line so that it can make a telephone call to a security company when the alarm is activated. The detector nodes, control panel nodes, and alarm nodes essentially comprise a Local Area Network (LAN).

In many of these security systems, the various nodes are connected to each other through wires. However, recently, such security systems are wireless systems. That is, each node includes a radio frequency (RF) transceiver and the nodes communicate with each other via low-power RF transmissions.

The Ademco™ technology developed by Honeywell International, Inc., is a radio chip set and a series of products that incorporate that chip set in conjunction with sensors is a wireless transceiver security system widely used throughout the United States and the world in wireless security systems such as those described above designed for use in home and business wireless security systems. It is used widely throughout the United States and the world in wireless security systems such as those described above. The Ademco technology includes wireless control panels, wireless detectors, and even wireless remote transmitters that can be placed within key fobs, watches, jewelry, or other personal items for remotely enabling or disabling the security system. For instance, a person might press a button on the remote unit when he or she arrives home, which will then transmit a unique code to the control node of the system instructing the system to disarm.

All of these features of the Ademco system could be useful in a system for monitoring and tracking the movements of individuals about a household, institution, or any other space.

The present invention is a method and apparatus for monitoring the location and movement of a person about a household or other space by having the person carry a wireless beaconing device that periodically transmits a beacon signal. FIG. 1 is a block diagram illustrating the basic components of a system 100 incorporating the present invention. In a preferred embodiment of the invention, the beaconing device 102 transmits a signal that includes (or possibly solely comprises) a unique ID (although the unique ID would not be necessary if only one person is to be tracked in any given household). The household or other space is equipped with a plurality of wireless receivers 104 (hereinafter anchors or anchor devices) for receiving the signals transmitted by the beaconing device 102. The anchors 104 should remain stationary once installed. Each time the beaconing device 102 sends out a signal and it is received by one or more of the anchor devices 104, the anchor devices record the ID of the beaconing device and determine and store the received signal strength. The various IEEE 802.11 specifications provide an exemplary technique for measuring RSSI (Received Signal Strength Indicator) for a received radio signal. This technique would be one way to determine received signal strength. However, the received signal strength may be determined in any reasonable fashion.

Each anchor device that receives a beaconing signal sends the ID of the beaconing device and its determined signal strength to a controller 106 at a control node of the wireless network 100. The anchor also should send a signal uniquely identifying the anchor unit that is transmitting the information. The control node may comprise any reasonable computing device, such as a microprocessor, PC, ASIC, state machine, processor, combinational logic, and any combination of software and hardware. The control node 106 correlates the information from the various anchor nodes and calculates an estimate of the position of the beaconing device. This process is repeated every time the beaconing device 102 transmits its signal.

The controller 106 preferably is pre-programmed with the location of each anchor node within the space being monitored so that it can translate the information received from the anchor devices into a physical location.

The control node 106 maintains a continuous record of the estimated location of the person. From this record, the movement of the person over a period of time can be determined relatively accurately. In one embodiment of the invention, the controller 106 may simply store this information for later retrieval by a caregiver. The controller may provide this information to the caregiver in any reasonable form, such a list of the start and end time of the tracked person in each room or a map showing a trail of the movement of the tracked person with or without time stamps. This information can be used to determine whether the person is moving about in a normal or expected fashion. It can also be used to determine if a person is going places within the space that he or she should not be.

In a preferred embodiment of the invention, a plurality of anchor nodes is positioned throughout the household. In one embodiment of the invention, one anchor node may be positioned in each room of the household. In other embodiments, particularly smaller households or systems using an algorithm that can accurately estimate the location of a beaconing device with fewer anchor nodes, there may not be a need for an anchor device in every room.

FIG. 2 is a block diagram illustrating a system in accordance with the principles of the present invention installed in a single level home. In this example, the home 200 comprises a garage 202, a kitchen 204, an entryway 206, a dining room 208, a living room 210, and two bedrooms, 212, and 214. Each room includes an anchor device 104. A control unit is positioned in the bedroom 212. The system includes one or more wireless beaconing devices 102 carried on the person or persons to be monitored. Preferably, all communication between nodes of the network is wireless.

FIG. 3 is a block diagram illustrating the basic components of an exemplary beaconing devices 102. The beaconing device should contain minimal signal processing capabilities so that it can be made as small and light weight as possible whereby it can be easily worn or carried by the monitored individuals. The beaconing device contains signal processing circuitry 302 for generating the signal to be transmitted. It further comprises transmitter circuitry 304 for conditioning the signal for RF transmission. Merely as an example, the transmitter circuitry 304 typically might include circuitry for converting the signal from digital to analog form, frequency up-converting the signal to RF and other signal conditioning circuitry that would be well within the understanding of those of skill in these arts. The unit further includes a transmission antenna 306. The signal processing circuitry 302 and transmitter circuitry 304 may be provided by one or more ASICs, microprocessors, analog hardware, digital hardware or any other reasonable technology. The transmit circuitry outputs the transmit signal to an antenna 306 for transmission. The unit should be powered by a long-life, small, lightweight battery 310.

Preferably, each beaconing transmits a binary signal that uniquely identifies that device. The system 100, of course, will be programmed to know what individual is carrying that particular device so as to be able to identify the individual from the particular ID.

FIG. 4 is a block diagram illustrating the basic components of an exemplary anchor device 104. The anchor device includes a receiving antenna 402 and RF processing circuitry 404 coupled to the antenna for extracting the signal received from the beaconing devices. Circuitry 404 typically would include circuitry for frequency down converting the received RF signal to a baseband signal and converting it from analog to digital. Anchor device 104 further comprises signal processing circuitry 406 for at least determining the received signal strength. In a preferred embodiment, circuitry 406 also determines the particular ID received. The anchor device also includes transmit circuitry 408 and a transmit antenna 410 for transmitting the signal strength information and/or ID information to the control node. The receive and transmit antennas, of course, may be the same single antenna.

FIG. 5 is a block diagram of the basic components of the control node 106 of the system. It includes a receiving antenna 502. It also includes receiver circuitry 504 for extracting the signal strength and/or ID information received from the anchor nodes 104 and converting it to baseband digital signals. It further includes a processor 506 for analyzing the data received from the anchor nodes 104 in order to estimate the location of the one or more beaconing devices based on that information. It includes a memory 508 for storing that information over time so as to be able to construct the movement of the beaconing devices over time and process that data to create a log or map of the movement of the beaconing device(s) over time. Furthermore, although not particularly relevant to the principles of the present invention, the control node likely also includes transmit circuitry 510 and a transmit antenna 512 for sending signals and information to the anchor nodes. Particularly, the control node 106 will include programming for running the entire network. Such functionality typically would require that the controller not only be able to receive information from the anchor nodes, but also transmit information to them. For instance, the controller may periodically test anchor nodes to make sure they are operating properly. Also, it may occasionally the send new software to the anchoring nodes.

Most of the functionality described below is performed by the processor 506 of the controller, which can take on any reasonable form, such as a signal processor, a programmed microprocessor, a programmed PC, digital hardware, analog hardware, a state machine, combinations thereof, etc. The various functionalities may be referred to herein as steps or circuits. It should be understood that these terms are used generically and are not intended to denote any particular hardware or software for performing the functionalities.

Various algorithms can be employed for estimating the location of the monitored individual based on the received signal strength. For instance, in one embodiment of the invention, the system can make a relatively broad determination of the instantaneous location of the person by simply deciding that the person is closest to the anchor device that reports the strongest signal strength. For example, if there is an anchor device in each room, then the person can be assumed to be in the room of the anchor device receiving the strongest signal. In many instances, this will be sufficient information for reasonably monitoring the individual.

However, if more precise estimation is desired, a more complex algorithm for estimating the location of the person can be employed. For instance, it may be possible to record the precise time of receipt of the signal at each anchor device and compare those times of receipt to each other to determine the differences between times of receipt and then trilaterate the position of the person based on that information. This technique would not use signal strength at all, but merely delay. In even further embodiments of the invention, an algorithm that uses both received signal strength and delay can be implemented.

In another embodiment of the invention, an algorithm can be used that considers the relative signal strengths recorded by multiple anchoring devices and trilaterates the position on the person based on those relative signal strengths. Below we describe a technique for accurately estimating the location of a person based at least partially on the signal strengths of the beacon signal as received at multiple locations, such as multiple anchor devices.

Generally, the weaker the received signal strength, the further away the beacon is from that particular anchor. FIG. 6 is a diagram illustrating distance as a function of received signal strength. Particularly, the points shown in FIG. 6 are actual data points obtained by empirical measurement. Solid line 601 illustrates the extrapolated average of all of the data points in the 5 feet to 50 feet range. Line 601 essentially defines the conversion from received signal strength to an estimated distance between the anchor device and the beacon.

It has been determined that highly accurate beacon location estimates can be obtained by solving an optimization problem to minimize the squared error over all of the anchors that received the signal from the beacon between (1) the predicted distance between the anchor and the beacon and (2) the distance between the known coordinates of the anchor and the estimated coordinates of the beacon.

FIG. 7 is a diagram illustrating the relative positions of a single beaconing device 701 and six anchor devices 702 ₁ through 702 ₆. In this example, we assume a two-dimensional space, which is perfectly adequate for a single level house in which all of the anchors as well as the beacon are highly likely to be at essentially the same elevation (e.g., within about 2 feet of each other). However, as will be discussed further below, the present technique is easily extendable to three dimensions and therefore, easily implemented in a multi-floor house or other structure also.

The aforementioned error between (1) the distance estimate between any given anchor and the beacon, on the one hand, and the distance between the known coordinates of that anchor and the estimated coordinates of the beacon can be expressed as: J _(i)=√{square root over ((X _(i) −X _(B))+(Y _(i) −Y _(B))²)}−d_(i) where

i=an index identifying the particular anchor (I=1 through 6 in this example having 6 anchors);

X_(B), Y_(B)=the Euclidian coordinates of the beacon;

X_(i), Y_(i)=the Euclidean coordinates of anchor i; and

d_(i)=the distance estimate based on the signal strength of the received signal at anchor i.

Thus, the minimization problem can be expressed as: $\min\limits_{X_{B},Y_{B}}{\sum\limits_{i = 1}^{n}J_{i}^{2}}$ where

n=the number of anchors receiving the beacon signal.

In other words, the estimated value for X_(B), Y_(B) that yields the smallest value of J₁+J₂+J₃+J₄+J₅+J₆ is the solution to the coordinates X_(B), Y_(B) of the beacon.

While this algorithm is adequate for many circumstances, in a preferred embodiment of the invention, the accuracy of the estimated location X_(B), Y_(B) of the beacon can be significantly increased by multiplying each squared error, J², by a weighting factor, w_(i). The weighting factor would be assigned based on the likely accuracy of the distance estimate for the particular anchor.

For instance, as illustrated in FIG. 6, in general, the weaker the received signal strength, the further away the beacon is from that particular anchor. However, in addition, the weaker the received signal strength (i.e., the further the beacon is from the particular anchor), the less accurate the distance estimate, d_(i). This also can be seen in FIG. 6. Particularly, as previously noted, the points shown in FIG. 6 are actually data points obtained by empirical measurement and solid line 601 is the extrapolated average distance as a function of received signal strength plotted from those data points. In addition, dashed lines 602 and 603 represent the minimum and maximum actual distances possible for a given signal strength based on the actual measurement data.

It can be seen that a received signal strength of 700 (the signal strength numbers are relative and, therefore, the units are insignificant) results in an estimated distance of approximately 8 feet (based on the average line 601), a signal strength of 600 results in a distance estimate of approximately 20 feet, and a signal strength of 550 results in a distance estimate of approximately 40 feet. The graph of FIG. 6 also illustrates that the accuracy of the predicted distance varies with distance. Particularly, the accuracy decreases as distance increases. For instance, at a signal strength of 700, it can be seen from the minimum and maximum lines 602 and 603, respectively, that the actual distance to the beacon is somewhere between 5 feet and 18 feet. This is an accuracy range of about 13 feet. However, when the signal strength drops down to 600, it can be seen that the actual distance to the beacon is somewhere between about 16 feet and about 42 feet. This is an accuracy range of 26 feet. Thus, the accuracy of the estimated distance when the signal strength is 600 is about half the accuracy of the estimated distance when the signal strength is 700. Thus, an anchor reporting a received signal strength of 600 should be assigned a weighting factor w_(i) that is about half the weighting factor assigned to an anchor reporting a received signal strength of 700.

Thus, with a weighting factor incorporated into the algorithm, the minimization equation for solving for the estimated distance, X_(B), Y_(B) of the beaconing device in accordance with the preferred embodiment of the invention can be expressed as: $\min\limits_{X_{B},Y_{B}}{\sum\limits_{i = 1}^{n}{w_{i}J_{i}^{2}}}$ where w_(i)=the weighting factor for anchor i.

In a simple embodiment of the invention that has proven to provide quite accurate location estimates of the beaconing device, the weighting factor may be a linear function of the estimated distance, d_(i).

The algorithm is easily extended to three dimensions for a multilevel house or any other environment in which elevation is a factor. Particularly, in three dimensions, the optimization problem is: $\min\limits_{X_{B},Y_{B},Z_{B}}{\sum\limits_{i = 1}^{n}{w_{i}J_{i}^{2}}}$ where J _(i)=√{square root over ((X _(i) −X _(B))²+(Y _(i) −Y _(B))²+(Z _(i)−Z_(B))²)}−d_(i)

Various computer-implemented algorithms for solving the optimization problem are well known in the related arts and need no further discussion.

Furthermore, it is advisable to add a further constraint, if possible, that the estimated location of the person is within known physical boundaries of the building (or other space). For instance, in a two floor house in which the first floor is at elevation 0 and the second floor is at elevation 10 feet, if the estimate provided by the present technique estimates that the person is at some other elevation, e.g., elevation 25 feet, the estimate should be corrected to the nearest realistic elevation, e.g., 10 feet (within some reasonable tolerance, such as 7-13 feet). The same rules can be applied in the horizontal directions, X and Y also. For instance, if there is a portion of the building that is clearly inaccessible to the tracked person (e.g., locked HVAC rooms, inside of walls, etc.), a constraint should be added to the algorithm that the estimated location cannot be an inaccessible location. Achieving this constraint is a simple matter of running the above discussed minimization algorithms and discarding any minimum values for X_(B), Y_(B), Z_(B) that do not meet the physical constraints until the lowest set of coordinates that meet the physical constraints is found. Another simple way to achieve this result, is to change any estimated location determined using the basic algorithm discussed above that is in an inaccessible space to the nearest coordinates that are within the accessible space.

The present invention can be implemented in connection with any plurality of distance estimates and is not limited solely to use in connection with distance estimates based on received signal strength. The above-noted algorithms can be used in connection with any system that provides distance estimates between a transmitter whose location is to be estimated and two or more receivers at known locations. In fact, the invention can even be applied to a system employing two or more different types of receivers at different locations.

For instance, as previously noted, in an alternative embodiment of the invention, one or more of the anchor devices may estimate distance between that anchor and the beacon based on a measured delay in the receipt of the beaconing signal. For instance, the delay may be a relative delay as measured at various anchor devices. Alternatively, the beacon signal may contain a timestamp that can be compared to the time of arrival at the anchor to determine the delay between the beaconing device sending out the signal and particular anchor device's receipt of the signal. Other well-known techniques for estimating distance include ultrasound. Thus, for purposes of example, let us assume a system that includes a first plurality of anchor devices that estimate distance to the beaconing device by received signal strength, a second plurality of anchor devices that estimates distance to the beacon utilizing time of arrival delay of the RF signal, and a third plurality of anchor devices that estimate the distance to the beaconing device utilizing the time of arrival of an ultrasound signal.

The above-noted algorithms can be used in such a system without modification. The different anchors simply would be assigned different weighting factors. For instance, distance estimates based on time of arrival measurements of electromagnetic signals, such as RF signals, are likely to be substantially more accurate than distance estimates based on received signal strength. Accordingly, those anchors would be assigned a greater weighting factor. Anchors that estimate distance based on ultrasound time of arrival generally tend to be less accurate than estimates based on time of arrival of electromagnetic signals because the speed of sound can vary based on various factors such as atmospheric pressure, humidity, and temperature. Nevertheless such estimates still tend to be much more accurate than estimates based on received signal strength measurement. Accordingly, distance estimates provided by anchor devices using ultrasound would be assigned a weighting factor greater than the weighting factors assigned to anchors using received signal strength, but lower than the weighting factors assigned to anchors using RF signal time of arrival to estimate distance.

Another factor that may affect the accuracy of the estimated distance between the beacon and the known location of the anchor device is the accuracy to which the location of the anchoring device is known. There are many factors that may influence the accuracy to which the allegedly known location of an anchor device is known. For instance, in an embodiment of the invention in accordance with aforementioned patent application No. 11/______, entitled Method and Apparatus for Tracking Persons (Attorney Docket No. H0011714-0760/Outside Counsel Docket No. P 31037) in which the plurality of anchor devices are control panels of a security system positioned various rooms of a house, the location of the anchor device may itself be estimated. U.S. patent application Ser. No. 11/______ entitled Method and Apparatus for Installing and/or Determining the Position of a Receiver of a Tracking System (Attorney Docket No. H0011619-0760/Outside Counsel Docket No. P 31054), the disclosure of which is fully incorporated herein by reference, discloses a number of different ways by which the location of the anchor devices of a tracking system such as the one discussed herein may be determined and reported to the controller. Some of them are more accurate than others. For instance, as discussed in that patent application, the location of an anchor may be very precisely measured using a tape measure, laser measuring devices or the like. Alternately, the location of an anchoring device may have been determined using GPS, which is only accurate to within about 15 feet (and possibly much less depending on the number of satellites being tracked by the GPS receiver used to determine the location of the anchor device. In another embodiment disclosed in that application, the location of an anchor device is determined by the use of a tracking device comprising an electronic compass, and a pedometer. More particularly, a person installing the anchor devices starts at a known, base location and walks with the tracking device (and the anchor device) while the tracking device counts the number of steps (using the pedometer calibrated to that person's stride) and tracks the direction (using the compass) of the person's movement. When the person reaches the location where the anchor will be installed, the person presses a button on the tracking device, which activates the tracking device to calculate its present location relative to the base location based on the number of steps and directions of those steps and wirelessly report that location to the system controller or to the anchor device being installed (e.g., for later reporting to the controller by the anchor device).

It should be obvious that all of these methods of determining the locations of the anchor devices inherently contains some error, some more than others. Accordingly, if one can determine, through empirical observation or otherwise, the relative accuracies of the various methods by which the purportedly known locations of the anchor devices can be determined, the weighting factor, w_(i), applied to the distance estimates provided by that anchor device also can be a function of that method.

The weighting factor may be set as a function of any one or more of the above-mentioned factors that affects the accuracy of the distance measurement. In fact, the weighting factor can be set partially or wholly as a function of any other factors that bear upon the accuracy of the distance measurements provided by the anchor devices and/or the accuracy to which the position of the anchor itself is known.

Having thus described a few particular embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements as are made obvious by this disclosure are intended to be part of this description though not expressly stated herein, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and not limiting. The invention is limited only as defined in the following claims and equivalents thereto. 

1. A method of estimating the location of an object based on estimates of distance between said object and a plurality of known locations, said method comprising the steps of: obtaining estimates of the distance between said object and a plurality of known locations; assigning a weighting factor to each of said estimates, said weighting factor being a function of a predicted accuracy of said estimate; and determining said estimated location of said object to be a location that is selected from the group of; an estimated location that minimizes a weighted squared error, over said plurality of estimates, between (a) said distance estimate and (b) a distance between said corresponding known location and said estimated location of said object; and an estimated location that minimizes a weighted squared error, over said plurality of estimates, between (a) said distance estimate and (b) a distance between said corresponding known location and said estimated location of said object, and is within a set of predetermined physical boundaries.
 2. The method of claim 1 wherein said distance estimates are based on a received signal strength of a signal received from said object.
 3. The method of claim 2 wherein said signal is a radio frequency signal transmitted from said object.
 4. The method of claim 1 wherein at least a first and a second one of said distance estimates are based on different technique for estimating said distance.
 5. The method of claim 1 wherein each of said squared errors is multiplied by said weighting factor assigned to the corresponding estimated distance.
 6. The method of claim 5 wherein each said weighting factor is a function of at least a technique by which said estimated distance was rendered.
 7. The method of claim 5 wherein each said weighting factor is a function of at least the accuracy of the corresponding known location.
 8. The method of claim 7 wherein each said weighting factor is a function of the technique by which the corresponding known location was determined.
 9. The method of claim 5 wherein said distance estimates are based on a received signal strength of a signal received from said object and wherein said weighting factor is a function of at least said estimated distance.
 10. The method of claim 5 wherein said distance estimates are based on a received signal strength of a signal received from said object and wherein said weighting factor is a function of at least said signal strength.
 11. The method of claim 10 wherein each said weighting factor is inversely proportional to said corresponding estimated distance.
 12. The method of claim 10 wherein each said weighting factor is inversely proportional to said corresponding signal strength.
 13. The method of claim 1 wherein said estimated location is determined by solving: $\min\limits_{X_{B},Y_{B}}{\sum\limits_{i = 1}^{n}{w_{i}J_{i}^{2}}}$ where J _(i)=√{square root over ((X _(i) −X _(B))²+(Y _(i) −Y _(B))²)}−d_(i) i=an index identifying a particular one of said known locations; X_(B), Y_(B)=the estimated Euclidian coordinates of said object; X_(i), Y_(i)=the estimated Euclidean coordinates of known location i; d_(i)=said distance estimate corresponding to known location i; w=said weighting factor; and n=the number of said known locations.
 14. The method of claim 1 wherein said estimated location is determined by solving: $\min\limits_{X_{B},Y_{B},Z_{B}}{\sum\limits_{i = 1}^{n}{w_{i}J_{i}^{2}}}$ where J _(i)√{square root over ((X _(i) −X _(B))²+(Y _(i) −Y _(B))²°(Z _(i) −Z _(B))²)}−d_(i) i=an index identifying a particular one of said known locations; X_(B), Y_(B), Z_(B)=the estimated Euclidian coordinates of said object; X_(i), Y_(i), Z_(i)=the estimated Euclidean coordinates of known location i; d=said distance estimate corresponding to known location i; and n=the number of said known locations.
 15. An apparatus for estimating the location of an object based on estimates of distance between said object and a plurality of known locations comprising: a beaconing device adapted to wirelessly transmit a beacon signal; a plurality of anchor devices each adapted to be positioned at various locations throughout a space to be monitored, said anchor devices further adapted to detect said beacon signal transmitted by said beaconing device; a first circuit adapted to determine an estimated distance between each said anchor device and said beaconing device based on said receipt of said signal transmitted by said beaconing device; and a second circuit adapted to determine an estimated location of said beaconing device to be a location that is selected from the group of; an estimated location that minimizes the weighted squared error, over said plurality of estimates, between (a) said distance estimate and (b) a distance between a location of said anchor device and said estimated location of said beaconing device; and an estimated location that minimizes a weighted squared error, over said plurality of estimates, between (a) said distance estimate and (b) a distance between said corresponding known location and said estimated location of said object, and is within a set of predetermined physical boundaries.
 16. The apparatus of claim 15 wherein said anchor devices are further adapted to detect said signal transmitted by said beaconing device and determine strength of said signal as received by the anchor device, and wherein said distance estimates are based on said received signal strength.
 17. The apparatus of claim 16 further comprising: a controller adapted to receive said signal strengths transmitted from said plurality of anchor devices, and wherein said first and second circuits form part of said controller.
 18. The apparatus of claim 17 wherein said anchor devices are further adapted to wirelessly transmit said signal strengths to said controller.
 19. The apparatus of claim 17 wherein said beacon signal is a radio frequency signal transmitted from said object.
 20. The apparatus of claim 15 wherein at least a first and a second one of said anchor devices use different techniques for estimating said distance.
 21. The apparatus of claim 15 wherein said second circuit multiplies each of said distance estimates by said weighting factor assigned to the corresponding estimated distance.
 22. The apparatus of claim 21 wherein each said weighting factor is a function of at least a technique by which said estimated distance was rendered.
 23. The apparatus of claim 21 wherein each said weighting factor is a function of at least the accuracy to which the location of the corresponding anchor device is known.
 24. The method of claim 23 wherein each said weighting factor is a function of the technique by which the location of the corresponding anchor device was determined.
 25. The apparatus of claim 21 wherein said anchor devices are further adapted to detect said signal transmitted by said beaconing device and determine a strength of said signal as received by the anchor device, and wherein said distance estimates are based on said received signal strength and wherein said weighting factor is a function of at least said distance estimate.
 26. The apparatus of claim 15 wherein said second circuit determines said estimated location by solving: $\min\limits_{X_{B},Y_{B}}{\sum\limits_{i = 1}^{n}{w_{i}J_{i}^{2}}}$ where J _(i)=√{square root over ((X _(i) −X _(B))²+(Y _(i) −Y _(B))²)}−d_(i) i=an index identifying a particular one of said known locations; X_(B), Y_(B)=the estimated Euclidian coordinates of said object; X_(i), Y_(i)=the estimated Euclidean coordinates of known location i; d_(i)=said distance estimate corresponding to known location i; and n=the number of said known locations.
 27. The apparatus of claim 15 wherein said second circuit determines said estimated location by solving: $\min\limits_{X_{B},Y_{B},Z_{B}}{\sum\limits_{i = 1}^{n}{w_{i}J_{i}^{2}}}$ where J _(i)=√{square root over ((X _(i) −X _(B))²+(Y _(i) −Y _(B))²+(Z _(i) −Z _(B))²)}−d_(i) i=an index identifying a particular one of said known locations; X_(B), Y_(B), Z_(B)=the estimated Euclidian coordinates of said object; X_(i), Y_(i), Z_(i)=the estimated Euclidean coordinates of known location i; d_(i)=said distance estimate corresponding to known location i; and n=the number of said known locations.
 28. The apparatus of claim 15 wherein said beaconing device transmits said signal at intervals and wherein said apparatus further comprises: a third circuit adapted to track movement of said beaconing device based on said estimates over a multiplicity of said signals transmitted by said beaconing device. 